This invention relates to drills, and more specifically to drills intended for work on hard materials and materials with a high heat resistant alloys such as hardened tool steel, die steel and other similar steels or fiberglass and composites. The toughest drills are commonly made of carbide in spiral flute configuration. The pressure and heat resistance to which they are subjected usually calls for a simple geometry with a relatively wide angle of cutting edge. In the art, a wide angle of cutting edge or nose angle means an angle of more than 59 degrees as measured from the axis of the tool. A 59 degree angle between the cutting edge and the axis is felt by some to equalize the thrust and torsion to best advantage; that is the proper balance between the pressure required for feeding the drill through the metal versus the power required to turn the drill. The acuity of keenness of the cutting edge also called the lip angle is an important factor which determines the strength of the tool and its durability. The lip angle is a combination of the rake angle, that is the slant of the forward face of the tool in relation to its axis, and of the clearance angle, the slant of the underface of the cutting tool. A positive rake angle i.e. when the front face slants upwardly and backwardly increases the keenness of the tool but greatly reduces its strength. A clearance angle of 12 degrees is common in most drills. Another characteristic which is critical in drill design for work on hard material is the shape, depth and length of the flute or fluid portion. Any fluting tends to decrease the strength of the tool and its ability to withstand high pressure and high torques. Flat spade drills have a simplified geometry with a relatively short fluid portion and no rake angle. They constitute the toughest type of tool which is particularly indicated for working on hard material. Conventional spade drills however, are usually very unstable and require the predrilling of a small leading bore. The length of a conventional carbide spade drill seldom exceeds twice the spun diameter. They usually have a very low clearance angle which leads to very high pressure and consequently high temperature operation which make them particularly unsuitable for working with high heat resistance materials such as heat resistant alloys, fiberglass and composites.
There is an acute need for a tough drill which would have all the basic favorable characteristics of a spade drill, but would operate at low pressure and temperature, and could drill up to 70 rockwell material on a C-scale.